Pharmacokinetics: properties favouring dialysis: practice question

A Pharmacology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Pharmacology · Pharmacokinetics: properties favouring dialysis

Sample item

A 60-year-old woman on long-term lithium presents with coarse tremor, ataxia, confusion and a markedly raised serum lithium with acute kidney injury. Haemodialysis is planned. Which property of lithium makes it effectively removed by dialysis?

AVery high protein binding with a small molecular sizeBExtensive hepatic metabolism to water-soluble productsCHigh lipid solubility with extensive tissue sequestrationDSmall volume of distribution with negligible plasma protein binding

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Small volume of distribution with negligible plasma protein binding

Dialysis removes small, water-soluble molecules that are not protein bound and that stay mainly in the plasma-accessible compartment. Lithium is a small ion with no protein binding and a modest volume of distribution, so it is readily dialysable. Highly protein-bound, lipophilic or widely distributed drugs are poorly removed.

Option by option

A. Very high protein binding with a small molecular size
Protein-bound drug cannot cross the membrane, and lithium is not protein bound.
B. Extensive hepatic metabolism to water-soluble products
Lithium is not metabolised; it is eliminated unchanged by the kidney.
C. High lipid solubility with extensive tissue sequestration
Lipophilic drugs sequestered in tissue are not accessible to dialysis.
D. Small volume of distribution with negligible plasma protein binding (correct)
Correct. A low molecular weight, unbound, water-soluble drug crosses the dialysis membrane easily.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is D, Small volume of distribution with negligible plasma protein binding. Dialysis removes small, water-soluble molecules that are not protein bound and that stay mainly in the plasma-accessible compartment.

Why is "Very high protein binding with a small molecular size" not the answer?

Protein-bound drug cannot cross the membrane, and lithium is not protein bound.

Why is "Extensive hepatic metabolism to water-soluble products" not the answer?

Lithium is not metabolised; it is eliminated unchanged by the kidney.

Why is "High lipid solubility with extensive tissue sequestration" not the answer?

Lipophilic drugs sequestered in tissue are not accessible to dialysis.

What concept does this question test?

It tests Pharmacokinetics: properties favouring dialysis in Pharmacology. Lithium is a small ion with no protein binding and a modest volume of distribution, so it is readily dialysable. Highly protein-bound, lipophilic or widely distributed drugs are poorly removed.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Pharmacokinetics: properties favouring dialysis?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.